Apparatus for joining metal sheets by punch riveting or penetration staking (clinching)

ABSTRACT

Apparatus for joining metal sheets by punch riveting or penetration staking (clinching) with the die ( 10; 20 ), wherein the die ( 10; 20 ) at least partly has been coated with a carbon layer, preferably a “diamond-like carbon”-(DLC)-coating or a carbon layer applied by plasma-coating or sputtering.

[0001] In connection with apparatuses for joining metal sheets by punch riveting or penetration staking (clinching) the problem is existing that in any case extremely high forces have to be applied. Due to this, in many cases, and especially when working aluminum sheets a partial alloying is occurring at the punch riveting dies and at the dies and punches, which are used when penetration staking (clinching).

[0002] Starting out from these problems, it is the task to be solved by the present invention to improve the corresponding punch riveting dies and the dies and punches for clinching, respectively, in such a way that definitely lower forces are necessary for working and that the problem of the occurrence of partial alloying at the dies, i.e. a kind of cold-welding or “freezing”, is avoided. According to the invention, this task is solved by the feature that the die at least partially is coated with a carbon layer. Surprisingly such a coating is leading to the fact that about 20% lower forces are necessary for creating the same joint. Further, the problem related to the partial alloying of aluminum sheets safely is solved. Additionally the wear of the dies is drastically lower and even if a wear is occurring such wear generally only is leading to a wear of the coating such that the die after a suitable new coating still can be used and not has to be substituted by a new die.

[0003] To save costs when producing the coating, it can be preferred that merely the areas facing the point of joint of the die are coated with a corresponding carbon layer.

[0004] In connection with apparatuses for clinching or penetration staking, which additionally to the die are having a punch, it is especially preferred if the punch at least partly is coated with the corresponding carbon layer.

[0005] In this connection, the die, in many cases, is having an anvil with laterally movable jaws thereon, wherein in this case it is especially preferred that as well the anvil as the jaws at least partly are coated with the carbon layer.

[0006] An especially suitable coating has shown to be a carbon coating in the nature of a “diamond-like carbon”-(DLC)-coating.

[0007] In this connection it is especially preferred if the carbon layer is applied by plasma-coating or sputtering.

[0008] In the following, the present invention is more detailedly described with reference to the exemplary embodiments shown in the drawings. In the drawings show:

[0009]FIG. 1 a die according to the invention for punch riveting;

[0010]FIG. 2 a die according to the invention for penetration staking or clinching;

[0011]FIG. 3 the anvil of the die according to FIG. 2;

[0012]FIG. 4 the jaws of the die according to FIG. 2;

[0013]FIG. 5 the punch coated according to the invention and belonging to the die according to FIG. 2.

[0014]FIG. 1 is showing an embodiment according to the invention as a die for the punch riveting. The corresponding die 10 is having an annular smooth exterior surface 12 facing the metal sheet parts, which are to be joined by punch riveting. In the center of the annular surface 12 a die 14 is recessed, in which the joining point is formed during the punch riveting procedure.

[0015] As shown in FIG. 1 by the dash-dotted lines, the surface 12 and the die 14 according to the invention are especially coated with carbon. This coating is a so-called “diamond-like carbon”-(DCL)-coating. According to the invention, a plasma-coating with carbon or a coating obtained by sputtering of carbon can be used. All these methods produce an extremely thin crystalline carbon disposit on the object treated. As shown in FIG. 1, the corresponding coating can extend around the corner onto a partial surface of the lateral area of the die 10. It is however of importance that both area, which are contacting the metal sheet to be worked, are coated.

[0016]FIG. 2 is showing a die 20 for penetration staking or clinching. The die is consisting of a generally cylindrical anvil 22, which is shown in FIG. 3, too, by itself having rounded front edges. Around the anvil there are provided four jaws 21, which are mounted on the anvil 22 in such a way that when producing a clinching point with the procedure of clinching or penetration staking they can laterally move elastically in a biased way.

[0017] According to the invention, the upper (front) area of the anvil 22 is coated with the carbon coating DLC described above. Such coating in this connection can extend, too, onto the lateral area of the anvil 22.

[0018] The coated regions in FIG. 3 are shown by a dash-dotted overlying line.

[0019]FIG. 4 is showing the jaws 21 of the die 20, which are coated, too, according to the invention. In accordance with the invention, at least the upper (front) side of the jaws 21 has to be DLC-coated. It is recommended however to coat the jaws 21, which always are exposed to extremely high loads during the clinching or penetration staking completely in the above described manner with carbon as this is shown in FIG. 4 by the dash-dotted line.

[0020]FIG. 5 is showing a punch 30, which can cooperate with the die 20 for creating a joint. The punch is having especially a cylindrical projection 32, the diameter of which is smaller than the diameter of the opening formed by the jaws 21. According to the invention, this cylindrical projection 32 of the punch 30, too, is carbon DLC-coated in the above described manner. The coating here should at least extend to the front area of the cylindrical part 32 and a part of the following lateral area.

[0021] According to the invention, such coating can be applied, too, subsequently on already existing dies and punches. Further, the coating according to the invention after wear can be renewed such that instead replacing the corresponding tools simply a new coating is sufficient.

[0022] By the coating according to the invention, the necessary joining forces as well with clinching or penetration staking as with punch riveting, too, drastically can be decreased. Simultaneously the life of the tools is substantially increased almost no mistakes are occurring and there are no problems of partially alloying when working with aluminum sheets. In the following, corresponding test results are stated as examples:

[0023] The problem of “projecting rivets” in the past has led to a continuously increase of the placing pressure. It is not understandable anymore why over the time this has occurred. To achieve an acceptable result during the production, the placing pressure has been set to about 250 bar corresponding to about 49 kN. From the collected experiences, it was shown that this is the only possibility to achieve an acceptable riveted joint. The increased pressure for all components of the apparatus means an increased load leading to a premature wear of a number of components.

[0024] To reduce the production pressure in the past several possibilities had been tested. Such, for example, punch rivets had been waxed or different suppliers had been tested for the surface coating to achieve a positive result. Unfortunately these steps did not lead to a significant success. In case forming tools of steel are used as e.g. punch riveting dies for forming aluminum without the addition of antiseizing or greasing agents, i.e. for dry forming, then over the time an adherence or cold welding, respectively, of aluminum particles on the steel surface is occurring. This leads to an increase of the coefficient of friction and therefore reduces the flow behaviour of the material in the die. To achieve the same results with respect to the flowing of the material in a die having a rough surface compared with a die having a smooth surface, the necessary force to overcome the frictional resistance has to be increased significantly.

[0025] The test made in this example was to show the influence of a so-called DLC-coating on the die surface. DLC (diamond-like carbon) has the property to create a layer, which is very hard and is having very good properties with respect to the separation. If this layer is undamaged, there is no adherence of aluminum on the surface.

[0026] To prove this theory in the test, dies, having a DLC-coating, have been used. The placing pressure was reduced to about 210 bar, about 41 kN. Over three working days (corresponding to about 50.000 rivets placed per die), the influence was checked under circumstances close to production. All rivets placed in this term showed an o.k.-property, i.e. the rivet head was contacting the surface of the workpiece. To check whether this was merely an accidental result, used uncoated dies had been used. The first placing of a rivet with these dies showed a projecting rivet and showed the same with ten further riveted carriers. Subsequent placements of rivets using DLC-coated dies led to an o.k.-placing pattern.

[0027] The disadvantage of the DLC-coating is consisting in the fact that the layer over the time is showing wear and therefore the adherence of aluminum again is occurring. The remedy here can be a cyclic exchange of the dies as e.g. found out in the test every three days. Further, the possibility is existing to then remove the coating from the dies and renew the coating. Here a logistical procedure has to be created, which realizes the number of circulated dies in a cost-saving manner. It is estimated that the life of a recoated die is about 60% of a new coated die. This, however, has to be shown in the practical work.

[0028] With regard to the costs, a higher price per die has to be expected because of the comparatively high costs for the coating. This, however, has to be seen in the comparison with the increased life of the die, which leads to a decrease of costs occurring with the failure of a die. Further, the reducing of the necessary pressure has a positive effect on the life of the entire system including hydraulic components and other mechanical components. Numer Carriers Number Pressure Run of Graph Date Time (overall) Rivets/Die (bar) Process Control Remarks 17.07.00 16:00 3185485   0 210 small variation dies with DLC used 17.07.00 18:15 3186291  1612 210 small variation smooth run of the graph 18.07.00 09:15 3191749 12528 210 small variation small chip-offs of the coating 18.07.00 16:15 3194279 17588 210 small variation chip-offs in the bottom of the die increasing; cone of the die shows no chip-offs; dies with- out coating tested and rivets are projecting; dies with coating used placing picture o.k. again 19.07.00 09:00 3200493 30016 210 variation increasing workpieces still shiny on the side of the die 19.07.00 15:00 3202650 34330 210 variation increasing workpieces still shiny on the side of the die with small interruptions, cone in the die still shiny 20.07.00 07:30 3208700 46430 210 variation increasing workpieces still metallic shiny on the side of the die 20.07.00 12:00 3210300 49630 210 variation increasing sporadically projecting rivets 20.07.00 13:00 3210500 50030 210 small variation polishing of the die; wear of the surface at the outer edge of the die (partly metallically shiny) 20.07.00 18:00 3211577 52184 210 variation increasing no projecting rivets 21.07.00 07:45 3215800 60630 210 variation increasing partly projecting rivets

[0029] The tests were done with original dies, Article-No. A000T-47195, having a DLC-coating. When reaching 50.000 placings of rivets, the dies were removed and polished since on places where the coating was chipped-off aluminum had been adhered. After the polishing, the die at these places again was metallically shiny. The number of carriers were read on a overall counter. Uncoated dies needed a placing pressure of about 250 bar to create an acceptable setting picture of the punch rivet. 

1. Apparatus for joining metal sheets by punch riveting or penetration staking (clinching) having a die (10; 20), characterized in that the die (10; 20) at least partly is coated with a carbon layer.
 2. Apparatus according to claims 1, characterized in that the die (10; 20) in the area facing the point of joining is coated with a carbon layer.
 3. Apparatus according to claim 1 or 2, for penetration staking (clinching) with an additional punch (30), characterized in that the punch (30), too, at least partly is coated with a carbon layer.
 4. Apparatus according to claim 3, in which the die (20) is having an anvil (22) with jaws (21) movable laterally thereon, characterized in that as well the anvil (22) as the jaws (21) at least partly are coated with a carbon layer.
 5. Apparatus according to any of the claims 1 to 4, characterized in that the carbon coating is a “diamond-like carbon”-(DLC)-coating.
 6. Apparatus according to any of the claims 1 to 4, characterized in that the carbon layer is applied by plasma-coating or sputtering. 